In electrophotography, photoreceptors frequently comprise a charge-generating material and a charge-transport material, often, but not always, as separate layers. The materials are mounted on a conductive support, which serves as an electrical contact. Electromagnetic radiation impinging on the photoreceptor generates mobile electric charges, or a current.
For applications in electrophotography, particularly in microimaging or very high quality 1:1 electrophotographic reproduction, it is important for a photoreceptor to resist localized "dielectric breakdown". Such "breakdown" makes a small area of the photoreceptor, typically about 5 to about 25 micrometers across, appear light-struck because the photoreceptor is unable to hold a surface charge. Some highly sensitive photoreceptors, including some of the two-layer photoreceptors described above, suffer from an excessively high density of breakdown spots. Although it is believed to involve a localized leakage of electrical current through the photoreceptor, the mechanism of this breakdown is not entirely understood, nor is it possible to predict which materials will be least prone to it.
Polynuclear oxazoles and thiazoles have been used in electrophotographic photoreceptors in a single-layer format, which consists of a mixture of an oxazole or thiazole, a polymeric binder, and sometimes a sensitizing dye all in a thin layer of homogeneous solid solution. See, for example, U.S. Pat. No. 3,257,204 where non-bis compounds are used. Polynuclear oxazoles and thiazoles have also been used as optical brighteners because they fluoresce efficiently in the solid state. See, for example, articles by A. Reiser,Journal of the American Chemical Society, 94, 2414 (1972), "Fluorescence of Aromatic Benzoxazole Derivatives"; and H. Gold, "Fluorescent Brightening Agents", The Chemistry of Synthetic Dyes, Ed. by K. Venkataraman, Academic Press, New York, 1971, Vol. V, p. 648.